The Franciscan church at the western edge of the old town is unremarkable from outside but conceals one of the great hidden frescoes of Tuscany: the Cappella della Croce di Giorno, painted between 1410 and 1415 by Cenni di Francesco Cenni. The walls are covered in cycles of the Legend of the True Cross and stories of the Passion, vivid in their Sienese-influenced palette and almost completely intact.
The chapel is dim and easy to miss; ask the sacristan to switch on the lights. A walk from the Piazza dei Priori takes about ten minutes through Via San Lino.